Page 65 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

The metal ions in this solution will be chiefly copper and zinc. In the presence of sodium chlo­
ride, there is a large increase in the deposition rates of sulfur dioxide on the metal surface.
Eriksson, Johansson, and Strandberg (1993) suggest that this accelerated deposition rate could be
connected to electrochemical corrosion processes on the alloy surface where the anodic disso­
lution of zinc and copper is balanced by oxygen reduction; in nitrogen dioxide environments,
it is balanced by the formation of nitrous acid. No information is available to date that quantifies
the corrosion of copper with other atmospheric agents, such as peroxyacetyl nitrates (PAN) and
carbonyl sulfide (COSH). In some countries, such as Brazil, the levels of PAN are quite high
because of the change from gasoline to ethanol for automobile fuel. Further research is required
to determine f these compounds also show synergistic effects on the corrosion of copper.
i
I COMPOSITION OF PATINAS I N THE OUTDOOR ENVIRONMENT
It has often been assumed that the patina that formed on exposed copper roofs or bronze stat­
ues consisted primarily of malachite, despite the fact that Gustav Magnus (1802-70) voiced sus­
picions as early as the I86OS that the primary minerals might be copper sulfates. He was
followed by conservator Friedrich Rathgen (1862-1942), who came to the same conclusion in the
1890S. Nonetheless, textbooks on general inorganic chemistry published up to 1996 still mention
malachite as the principal product formed under these circumstances. It had long been known
that exposed copper roofs gradually develop a patina of cuprite and that this reddish patina
would imperceptibly turn green over long periods (thirty to seventy years or more) of exposure
to the atmosphere. No studies, however, had been carried out before World War I to establish
what minerals were actually formed or what corrosion processes were at work.
An impressive corpus of observations was made by a group of German scientists who inves­
tigated the deterioration of exposed bronze statuary in Berlin beginning in the 186OS and con­
tinuing through the i890s. Their series of experiments, initiated by the Berlin Society for the
Encouragement of the Arts and directed by Magnus, examined the effects of oil on the preser­
vation of exposed bronzes (Magnus 1864). This was the first serious acknowledgment that there
was a problem in this regard. Some of the results of that work are summarized by Hiorns (1892).
In the experiments, a series of bronze busts were placed in different areas of Berlin where the
air was polluted and bronze surfaces were turning black. The observers noticed, however, that
on the bronze busts that had been accessible to handling, an attractive green patina had formed;
they inferred that this alteration was due to the effects of human skin oils. In another series of
experiments, the group placed four test bronzes outdoors: one was washed daily and rubbed
thoroughly with bone oil once a month; the second was washed daily and rubbed with oil twice
a year; the third was washed daily with no application of oil; and the fourth was kept unwashed
and untreated as a control. The results of these experiments showed that the bronze washed
daily and oiled monthly developed a fine green patina; the bronze oiled twice a year had less
satisfactory appearance; the bronze that was washed but not oiled did not develop any green
patina; and the control was dirty and unattractive. The scientists recommended rubbing exposed

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